Recombinant polypeptides derived from FBP1 and FBP2 and uses of the same

ABSTRACT

Disclosed herein are recombinant polypeptides derived from FBP1 and FBP2. Also disclosed herein are recombinant expression vectors and recombinant host cells for producing the aforesaid recombinant polypeptides. The recombinant polypeptides are proven to be useful and effective in producing a picornavirus with a type I internal ribosome entry site (IRES), so as to facilitate the preparation of a viral vaccine.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Patent Application No. 106113165, filed on Apr. 19, 2017.

FIELD

The disclosure relates to recombinant polypeptides derived from FBP1 and FBP2. Such recombinant polypeptides are useful and effective in increasing viral yield.

BACKGROUND

Many RNA viruses utilize internal ribosome entry sites (IRESs) located in the 5′ untranslated region (UTR) of genomic RNA to translate viral proteins in a cap-independent manner. Several cellular host proteins that can bind to and stabilize IRES structures and regulate IRES-driven translation have been reported, and these proteins are known as IRES trans-acting factors (ITAFs). Far upstream element-binding protein 1 (FUBP1, also known as FBP1) and far upstream element-binding protein 2 (KHSRP, also known as KH-type splicing regulatory protein, KSRP, FUBP2, FBP2 or P75) are among the known ITAFs that are functionally important in modulating viral IRES-driven translation.

IRES elements have been shown to exist in all genera of Picornaviridae, a type of cytoplasmic RNA virus known for its infectability on both animals and humans. At present, at least five different types of IRESs have been identified in picornaviruses, and each type is characterized by a distinct secondary structure and a different eIF (eukaryotic initiation factor)/ITAF-requirement. The picornavirus type I IRES is only found in enteroviruses such as enterovirus A71 (EVA71 or EV71), coxsackievirus B3 (CVB3), and human rhinovirus type 2 (HRV2).

EV71 is a positive single-strand RNA virus containing type I IRES in the Picornaviridae family, and is emerging as a potent threat worldwide. EV71 infections normally cause mild diseases, such as hand-foot-and-mouth disease (HFMD) or herpangina. However, children under five years of age are particularly susceptible to the most severe forms of EV71-associated neurological complications, including aseptic meningitis, brainstem and/or cerebellar encephalitis, acute flaccid paralysis (AFP), myocarditis, and rapid fatal pulmonary edema and hemorrhage.

The applicants previously found that FBP2 is a negative regulator (Lin J. Y. et al. (2009), Nucleic Acids Res., 37:47-59), while FBP1 is a positive regulator of EV71 IRES-dependent translation (Huang P. N. et al. (2011), Nucleic Acids Res., 39:9633-9648). Moreover, when the C-terminal of FBP2 is cleaved upon EV71 infection, the cleaved form of FBP2 (FBP2¹⁻⁵⁰³) then becomes a positive regulator of EV71 IRES-driven translation. Ubiquitination, and proteasomal and lysosomal activities may be involved in EV71-induced FBP2 truncation (Chen L. L. et al. (2013), J Virol., 87:3828-3838).

By conducting research, the applicants surprisingly found that polypeptides derived from FBP1 and FBP2, respectively, can enhance viral IRES activity and increase viral yield. Therefore, these FBP1 and FBP2 polypeptides are expected to be useful in vaccine production.

SUMMARY

Therefore, according to a first aspect, the disclosure provides a recombinant polypeptide having an amino acid sequence corresponding to that of a truncated mutant product of a wild-type FBP1 protein having 644 amino acids in length, the truncated mutant product lacking a C-terminal domain of the wild-type FBP1 protein.

According to a second aspect, the disclosure provides a recombinant polypeptide having an amino acid sequence corresponding to that of a mutant product of a wild-type FBP2 protein having 711 amino acids in length, each of amino acid residues at positions 109, 121 and 122 of the wild-type FBP2 protein being lysine, at least one of amino acid residues at positions 109, 121 and 122 of the mutant product being arginine instead of lysine.

According to a third aspect, the disclosure provides a recombinant nucleic acid encoding a recombinant polypeptide as described above.

According to a fourth aspect, the disclosure provides a recombinant expression vector comprising a recombinant nucleic acid as described above.

According to a fifth aspect, the disclosure provides a recombinant cell comprising a recombinant expression vector as described above.

According to a sixth aspect, the disclosure provides a method for producing a picornavirus with a type I internal ribosome entry site (IRES), the method comprising the steps of:

providing a recombinant cell as described above;

infecting the recombinant cell with the picornavirus;

incubating the infected recombinant cell; and

harvesting the picornavirus produced.

According to a seventh aspect, the disclosure provides a method for preparing a viral vaccine using a harvested picornavirus as obtained by the method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawings, of which:

FIG. 1 shows the expression of FBP1 protein and its cleavage products in EV71 virus-infected human embryonal rhabdomyosarcoma (RD) cells at 2-10 hours post-infection (h.p.i.), in which viral 3D^(pol) protein was used as an indicator for virus infection, actin served as a loading control, and Cps represents the cleavage products of FBP1;

FIG. 2 shows the expression level of FBP1 protein and its cleavage products (Cp-N and Cp-C) in mock-infected or EV71-infected RD cells at 4, 6, 8 and 10 h.p.i. detected by Western blotting using two antibodies, Ab-N and Ab-C, that respectively recognize the N-terminal epitope (amino acid positions 61-80) and C-terminal epitope (amino acid positions 293-644) of FBP1, in which viral 3D^(pol) protein was used as an indicator for virus infection, actin served as a loading control, Cp-N and Cp-C respectively represent cleavage products including the N-terminal and C-terminal regions of FBP1, and CASP-Cp-N represents a caspase-induced cleavage product of Cp-N;

FIG. 3 shows the cleavage pattern of [³⁵S]methionine-labeled FBP1 induced by various doses of purified recombinant EV71 viral protease wild-type 2A^(pro) or the catalytic defective mutant 2A^(pro) (2A^(pro)-C110S) for 4 hours, in which Cp-N and Cp-C respectively represent cleavage products including the N-terminal and C-terminal regions of FBP1;

FIG. 4 shows the cleavage patterns of [³⁵S]methionine-labeled wild-type FBP1 and seven mutant FBP1 proteins labeled with [³⁵S] methionine (FBP1-G345-362K, FBP1-G364-380K, FBP1-G364K, FBP1-G366K, FBP1-G371K, FBP1-G374K and FBP1-G375K) induced by EV71 2A^(pro) for 4 hours, in which non-specific cleavage products are indicated with an asterisk, and Cp-N and Cp-C represent cleavage products of FBP1 respectively including the N-terminal and C-terminal regions of FBP1;

FIG. 5 is a schematic diagram showing predicted molecular weights of the cleavage products of wild-type FBP1 and five truncated FBP1 protein (FBP1¹⁻³⁷¹, FBP1³⁷²⁻⁶⁴⁴, FBP1¹⁻⁴⁴³, FBP1¹⁸⁵⁻⁶⁴⁴, and FBP1¹⁸⁵⁻⁴⁴³) at the proposed primary cleavage site at Gly-371 (indicated by an arrow), in which KH1 to KH4 represent the four K-homology (KH) domains of FBP1 for binding to DNA;

FIG. 6 illustrates the cleavage patterns of [³⁵S]methionine-labeled wild-type FBP1 and five truncated FBP1 protein labeled with [³⁵S] methionine shown in FIG. 5 as induced by EV71 2A^(pro);

FIG. 7 illustrates the immunoblotting results showing the expressions of FBP1 protein and its cleavage products in RD cells transfected with a plasmid expressing FLAG-HA dual-tagged wild-type FBP1 protein or mutant FBP1-G371K protein, which is resistant to 2A^(pro) cleavage, after infection with EV71 for 4, 6, 8 and 10 hours, in which Cp-N and Cp-C represent cleavage products of FBP1 respectively containing the FLAG-tagged N-terminal region and HA-tagged C-terminal region of FBP1, viral 3D^(pol) protein was used as an indicator for virus infection, actin served as a loading control;

FIG. 8 illustrates the result of pull-down assay showing an association between an RNA probe (i.e., biotinylated or nonbiotinylated EV71 5′UTR (EV71 nucleotide positions 1-745), or biotinylated or nonbiotinylated EV71 5′UTR linker region (EV71 nucleotide positions 636-745)) and the cell lysate containing FLAG-tagged wild-type FBP1 or truncated FBP1 protein (FBP1³⁷¹ or FBP1³⁷²⁻⁶⁴⁴)

FIG. 9 illustrates the result of RNA footprinting assay showing a mapping association of wild-type FBP1 and truncated FBP1¹⁻³⁷¹ protein with the EV71 5′ UTR linker region RNA probe at the nucleotide sequence level as obtained using RNase T1 and RNase A;

FIG. 10 illustrates the impact of various concentrations of wild-type FBP1 and truncated FBP1¹⁻³⁷¹ on EV71 IRES-driven translation activity in vitro, in which the symbols “*” and “**” respectively represent p<0.05 and p<0.01 as compared to the buffer control;

FIG. 11 illustrates the additive effect of wild-type FBP1 and truncated FBP1¹⁻³⁷¹ on EV71 IRES-driven translation activity in vitro, in which the symbol “**” represents p<0.01 as compared to FBP1;

FIG. 12 shows the viral titers titrated by plaque assays in shNC-RD stable cells or in shFBP1-RD stable cells transfected with a vector control or plasmids expressing FLAG-tagged wobble mutant FBP1 (FBP1^(WM)) or FBP1-G371K^(WM) after infection with EV71 at a m.o.i. of 40 for different time periods, in which the symbols “*” and “**” respectively represent p<0.05 and p<0.01;

FIG. 13 shows the viral titers titrated by plaque assays in shFBP1-RD stable cells transfected with recombinant plasmids expressing different mutant FBP1 proteins after infection with EV71 at a m.o.i. of 40 for 9 hours, in which the symbols “*”, “**” and “***” respectively represent p<0.05, p<0.01 and p<0.001;

FIG. 14 shows the viral titers titrated by plaque assays in shFBP1-RD stable cells transfected with recombinant plasmids expressing different mutant FBP1 proteins after infection with CVB3 at a m.o.i. of 40 for 9 hours, in which the symbols “*” and “#” represent p<0.05, and the abbreviation “n.s.” indicates non-significance;

FIG. 15 is a schematic diagram of predicted ubiquitination sites in FBP2;

FIG. 16 shows the ubiquitination patterns of wild-type FBP2 and seven FBP2 mutants represented by K109R, K251R, K628R, K646R, K654R, N-ter 5K5R and K109,121,122R in 293T cells co-transfected with a plasmid expressing HA-tagged ubiquitin (HA-Ub), in which the asterisk (*) represents the major modification (ubiquitination) of FBP2;

FIG. 17 shows the fold-changes in the major modification (ubiquitination) of wild-type FBP2 and three FBP2 mutants represented by K109,121,122R, K121,122R and K109R compared to that of wild-type FBP2 protein;

FIG. 18 shows the impact of various concentrations of wild-type FBP2 and FBP2-K109,121,122R on EV71 IRES-driven translation activity in vitro, in which the abbreviation “n.s.” indicates non-significance, and the symbol “**” represents p<0.01;

FIG. 19 shows the competitive capability of different concentrations of a recombinant FBP1 protein against wild-type FBP2 and FBP2-K109,121,122R in interacting with biotinylated EV71 5′UTR RNA probe;

FIG. 20 shows the relative expression levels of VP1 and 3C^(pro) against actin in shFBP2-RD stable cells overexpressing wild-type FBP2 and FBP2-K109,121,122R after infection with EV71 at 10 m.o.i. for 5-6 hours, in which the abbreviation “n.s.” indicates non-significance, and the symbol “**” represents p<0.01;

FIG. 21 shows the viral titers titrated by plaque assays in shFBP2-RD stable cells overexpressing wild-type FBP2 or each of four mutant FBP2 proteins (FBP2-K109,121,122R, FBP2-K109R, FBP2-K121R and FBP2-K122R) after infection with EV71 at m.o.i. of 40 for 9 hours, in which the symbol “*” represents p<0.05; and

FIG. 22 shows the viral titers titrated by plaque assays in shFBP2-RD stable cells overexpressing FLAG, wild-type FBP2 or FBP2-K109,121,122R after infection with CVB3 at m.o.i. of 40 for 9 hours, in which the abbreviation “n.s.” indicates non-significance, and the symbols “*” and “#” respectively represent p<0.05.

DETAILED DESCRIPTION

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Taiwan or any other country.

For the purpose of this specification, it should be clearly understood that the word “comprising” means “including but not limited to”, and that the word “comprise” has a corresponding meaning.

Unless otherwise defined, all technical and scientific terms used herein have the meaning as commonly understood by a person skilled in the art to which the present disclosure belongs. One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present disclosure. Indeed, the present disclosure is in no way limited to the methods and materials described.

The terms “polypeptide”, “peptide”, and “protein” as used herein can be interchangeably used, and refer to a polymer formed of amino acid residues, wherein one or more amino acid residues are naturally occurring amino acids or artificial chemical mimics. The term “recombinant polypeptide” or “recombinant protein” as used herein refers to polypeptides or proteins produced by recombinant DNA techniques, i.e., produced from cells transformed by an exogenous DNA construct encoding the desired polypeptide or the desired protein.

As used herein, an amino acid can be represented by the full name thereof, by the three-letter symbol corresponding thereto, or by the one-letter symbol corresponding thereto, as well-known in the art. In addition, the proteins are represented in accordance with the conventional way of describing peptides, that is, with the N-terminus (amino terminus) on the left side and the C-terminus (carboxyl terminus) on the right side.

A “wild-type” protein means that the protein will be active at a level of activity found in nature and typically will be the amino acid sequence found in nature. In an aspect, the term “wild type” or “parental sequence” can indicate a starting or reference sequence prior to a manipulation of this invention.

As used herein, the term “mutation” refers to a change introduced into a parental sequence, including, but not limited to, substitutions, insertions, and deletions (including truncations). The consequences of a mutation include, but are not limited to, the creation of a new character, property, function, phenotype or trait not found in the protein encoded by the parental sequence.

As used herein, the term “mutant” refers to a gene or gene product that displays modifications in sequence and/or functional properties (i.e., altered characteristics) when compared to the wild-type gene or gene product. It is noted that naturally-occurring mutants can be isolated; these are identified by the fact that they have altered characteristics when compared to the wild-type gene or gene product. The mutant may be one that exists in nature, such as an allelic mutant, or one not yet identified in nature. The mutant may be conservatively altered, wherein substituted amino acid(s) retain structural or chemical characteristics similar to those of the original amino acid(s). Rarely, mutants may be substituted non-conservatively.

The term “truncated product” with reference to a protein, polypeptide or fragment thereof generally denotes such product that has a N-terminal and/or C-terminal deletion of one or more amino acid residues as compared to said protein, polypeptide or fragment thereof.

A “DNA coding sequence” is a double-stranded DNA sequence that is transcribed into an RNA (further translated into a polypeptide) in vivo under the control of appropriate regulatory sequences. The boundaries of the DNA coding sequence are determined by a start codon at the 5′ (amino) terminus and a stop codon at the 3′ (carboxyl) terminus. A coding sequence may include, but is not limited to, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic (e.g., mammalian) DNA, and even synthetic DNA sequences. “cDNA” is defined as copy DNA or complementary DNA, and is a product of a reverse transcription reaction from an mRNA transcript.

The terms “nucleic acid”, “nucleic acid sequence”, and “nucleic acid fragment” as used herein refer to a deoxyribonucleotide or ribonucleotide sequence in single-stranded or double-stranded form, and comprise naturally occurring nucleotides or artificial chemical mimics. The term “nucleic acid” as used herein is interchangeable with the terms “gene”, “cDNA”, “mRNA”, “oligo-nucleotide”, and “polynucleotide” in use.

As used herein, the term “DNA fragment” refers to a DNA polymer, in the form of a separate segment or as a component of a larger DNA construct, which has been derived either from isolated DNA or synthesized chemically or enzymatically such as by methods disclosed elsewhere.

Unless otherwise indicated, a nucleic acid sequence, in addition to the specific sequences described herein, also covers its complementary sequence, and the conservative analogs, related naturally occurring structural variants and/or synthetic non-naturally occurring analogs thereof, for example, homologous sequences having degenerative codon substitution, and conservative deletion, insertion, substitution, or addition. Specifically, degenerative codon substitution may be produced by, for instance, a nucleotide residue substitution at the third position of one or more selected codons in a nucleic acid sequence with other nucleotide residue(s).

The terms “recombinant vector” and “expression vector” as used herein can be interchangeably used, and refer to any recombinant expression system capable of expressing a selected nucleic acid sequence, in any competent host cell in vitro or in vivo, constitutively or inducibly. The recombinant vector may be an expression system in linear or circular form, and covers expression systems that remain episomal or that integrate into the host cell genome. The recombinant expression system may or may not have the ability to self-replicate, and it may drive only transient expression in a host cell.

As used herein, the term “transformation” can be used interchangeably with the term “transfection” and refers to the introduction of an exogenous nucleic acid molecule into a selected host cell. According to techniques known in the art, a nucleic acid molecule (e.g., a recombinant DNA construct or a recombinant vector) can be introduced into a selected host cell in various ways, such as calcium phosphate- or calcium chloride-mediated transfection, electroporation, microinjection, particle bombardment, liposome-mediated transfection, transfection using bacteriophages, or other methods.

The terms “cell”, “host cell”, “transformed host cell”, and “recombinant host cell” as used herein can be interchangeably used, and not only refer to specific individual cells but also include sub-cultured offsprings or potential offsprings thereof. Sub-cultured offsprings formed in subsequent generations may include specific genetic modifications due to mutation or environmental influences and, therefore, may factually not be fully identical to the parent cells from which the sub-cultured offsprings were derived. However, sub-cultured cells still fall within the coverage of the terms used herein.

In order to increase viral yield for use in vaccine production, the applicants endeavored to develop improved methods and found that polypeptides derived from FBP1 and/or FBP2 can enhance viral IRES activity and increase viral yield.

Accordingly, the present disclosure provides a first recombinant polypeptide having an amino acid sequence corresponding to that of a truncated mutant product of a wild-type FBP1 protein having 644 amino acids in length. The truncated mutant product lacks a C-terminal domain of the wild-type FBP1 protein.

The term “corresponding to” is used herein to describe a polypeptide or nucleic acid of this disclosure which is similar or homologous to a corresponding mutant product modified from a parental polypeptide or nucleic acid, wherein the sequence of the polypeptide or nucleic acid of this disclosure differs from the sequence of the corresponding mutant product modified from the parental polypeptide or nucleic acid only by the presence of at least one amino acid residue or nucleotide residue variation.

Typically, the sequences of the polypeptide or nucleic acid of the disclosure and the corresponding mutant product modified from the parental polypeptide or nucleic acid exhibit a high percentage of identity, such as at least about 70%, at least about 80%, at least about 90%, at least about 95%, at least about 98% identity, or at least about 99% identity.

In certain embodiments, the truncated mutant product of the wild-type FBP1 protein has amino acid deletions at positions 372-644 of the wild-type FBP1 protein. In an embodiment of the present disclosure, the truncated mutant product of the wild-type FBP1 protein has an amino acid sequence of SEQ ID NO: 4.

In certain embodiments, the truncated mutant product of the wild-type FBP1 protein has amino acid deletions at positions 63-78 and positions 372-644 of the wild-type FBP1 protein. In an embodiment of the present disclosure, the truncated mutant product of the wild-type FBP1 protein has an amino acid sequence of SEQ ID NO: 46.

The present disclosure also provides a first recombinant nucleic acid encoding the first recombinant polypeptide as described above. In certain embodiments, the first recombinant nucleic acid has a nucleotide sequence selected from SEQ ID NO: 3, SEQ ID NO: 44 and SEQ ID NO: 45.

In addition, the present disclosure also provides a second recombinant polypeptide having an amino acid sequence corresponding to that of a mutant product of a wild-type FBP2 protein having 711 amino acids in length. Each of amino acid residues at positions 109, 121 and 122 of the wild-type FBP2 protein is lysine, and at least one of amino acid residues at positions 109, 121 and 122 of the mutant product is not lysine.

In certain embodiments, the at least one amino acid residue at positions 109, 121 and 122 of the mutant product is arginine. In an exemplary embodiment, one amino acid residue at positions 109, 121 and 122 of the mutant product is arginine. In an exemplary embodiment, two of the amino acid residues at positions 109, 121 and 122 of the mutant product are arginine. In yet another exemplary embodiment, all of the amino acid residues at positions 109, 121 and 122 of the mutant product are arginine.

In an embodiment of the present disclosure, the amino acid sequence of the second recombinant polypeptide is selected from the group consisting of SEQ ID NO: 8, SEQ ID NO: 10, SEQ ID NO: 12, SEQ ID NO: 14 and SEQ ID NO: 16.

The present disclosure also provides a second recombinant nucleic acid encoding the aforesaid second recombinant polypeptide derived from FBP2. In certain embodiments, the second nucleotide sequence is selected from the group consisting of SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11, SEQ ID NO: 13 and SEQ ID NO: 15.

The first and second recombinant nucleic acid sequences according to the present disclosure can be utilized to construct various recombinant expression vectors for expressing the aforementioned recombinant polypeptides using a standard technique known to one of ordinary skill in the art.

Therefore, the present disclosure provides a recombinant expression vector comprising the first recombinant nucleic acid and/or the second recombinant nucleic acid.

In certain embodiments, the first and/or second recombinant nucleic acids of the recombinant expression vector may be introduced into a genome of the recombinant cell. In an exemplary embodiment, an endogenous FBP2 gene in the genome of the recombinant cell is deleted, disrupted, or disabled.

As used herein, the term “delete” refers to partial or entire removal of a coding region of a gene.

As used herein, the term “disrupt” refers to removal, insertion, or mutation of a nucleotide of a gene.

As used herein, the term “disable” refers to inactivating a gene or the protein encoded by the gene so as to force the gene or protein to lose its activity or function.

The recombinant expression vector according to the present disclosure can be used to transform or transfect a desired host cell. Consequently, the disclosure also provides a recombinant cell comprising the aforementioned recombinant expression vector.

According to the disclosure, the host cell may be a mammalian cell. Examples of the mammalian cell suitable for used in the present disclosure include, but are not limited to, an RD cell, a 293T cell, a Vero cell, a MDCK cell, a PER.C6 cell, a MRC-5 cell, a WI-38 cell, etc.

Since the aforementioned recombinant polypeptides derived from FBP1 and/or FBP2 can promote picornavirus type I IRES-driven translation and increase viral yield, these polypeptides are expected to be useful in the production of viral vaccine. Therefore, the present disclosure provides a method for producing a picornavirus with a type I IRES, which includes the steps of:

providing the aforesaid recombinant cell;

infecting the recombinant cell with the picornavirus;

incubating the infected recombinant cell; and

harvesting the picornavirus produced.

According to the disclosure, the picornavirus with the type I IRES may be an enterovirus selected from the group consisting of enterovirus A (such as enterovirus A71 and coxsackieviruses A6 and A16), enterovirus B (such as coxsackievirus B3), enterovirus C (such as coxsackieviruses A21 and A24, and polioviruses 1, 2 and 3), enterovirus D (such as enterovirus D68), rhinovirus A, rhinovirus B, and rhinovirus C, and combinations thereof. In an embodiment of the disclosure, the enterovirus is enterovirus A71. In another embodiment of the disclosure, the enterovirus is coxsackievirus B3.

The disclosure also provides a method for preparing a viral vaccine using the harvested picornavirus obtained by the aforesaid method for producing the picornavirus.

The present disclosure will be further described by way of the following examples. However, it should be understood that the following examples are intended solely for the purpose of illustration and should not be construed as limiting the disclosure in practice.

EXAMPLES

General Experimental Materials:

1. Primers used in the polymerase chain reaction (PCR) experiments, infra, were synthesized by Mission Biotech, Taipei, Taiwan.

2. EV71 strain Tainan/4643/98 viruses (GenBank accession number AF304458) and CVB3 strain viruses were obtained from Chang Gung Memorial Hospital, Linkou, Taiwan.

3. Cell Cultures:

Human embryonal rhabdomyosarcoma (RD) cells (ATCC, CCL-136) and 293T cells (ATCC, CCL-3216) used in the following experiments were purchased from ATCC (American Type Culture Collection, Manassas, Va., USA). The cells of the respective type were incubated in a Petri dish containing Dulbecco's modified Eagle medium (DMEM; Gibco, Grand Island, N.Y.) supplemented with 10% fetal bovine serum (FBS; Gibco), 100 units/mL of penicillin, 100 μg/mL of streptomycin and 0.25 μg/mL of amphotericin B, followed by cultivation in an incubator with culture conditions set at 37° C. and 5% CO₂. Medium change was performed every two days. Cell passage was performed when the cultured cells reached 90% of confluence.

4. FBP1-Related Recombinant Plasmids:

-   (a) Plasmid pFLAG-CMV2-FBP1 carrying, amongst others, a FBP1 coding     sequence (SEQ ID NO: 1, GeneBank accession no. KY569017) encoding a     wild-type FBP1 protein having an amino acid sequence of SEQ ID NO:     2, was generated as follows. The FBP1 coding sequence was amplified     from a plasmid pCMV-tag2B-FBP1 (as described in Huang P. N. et al.     (2011), supra) using a primer pair FBP1-F1 and FBP1-R1 as shown in     Table 1. The resultant FBP1 coding sequence was then in-frame     inserted into the pFLAG-CMV2 vector (Sigma, E7033) at the NotI and     EcoRV sites. -   (b) For obtaining five C-terminal and/or N-terminal truncated FBP1     proteins, i.e. FBP1¹⁻³⁷¹ (having an amino acid sequence of SEQ ID     NO: 4), FBP1³⁷²⁻⁶⁴⁴, FBP1¹⁻⁴⁴³, FBP1¹⁸⁵⁻⁶⁴⁴, and FBP1¹⁸⁵⁻⁴⁴³, each     of their corresponding coding sequences was amplified from     pFLAG-CMV2-FBP1 using the respective primer pairs as shown in     Table 1. A respective one of these coding sequences was in-frame     inserted between the NotI and EcoRV sites of the pFLAG-CMV2 vector,     so as to obtain five recombinant plasmids, i.e.,     pFLAG-CMV2-FBP1¹⁻³⁷¹, pFLAG-CMV2-FBP1³⁷²⁻⁶⁴⁴, pFLAG-CMV2-FBP1¹⁻⁴⁴³,     pFLAG-CMV2-FBP1¹⁸⁵⁻⁶⁴⁴ and pFLAG-CMV2-FBP1¹⁸⁵⁻⁴⁴³.

TABLE 1 (Full or Truncated) SEQ FBP1 coding Primer ID sequences Pair The primer's nucleotide sequence (5′→3′) NO. FBP1 FBP1- AAGCTTGCGGCCGCGATGGCAGACTATTCAACA 17 F1       NotI FBP1- GGTACCGATATCAGTTATTGGCCCTGAGGTGC 18 R1       EcoRV FBP1¹⁻³⁷¹ FBP1- AAGCTTGCGGCCGCGATGGCAGACTATTCAACA 17 F1       NotI FBP1- GGTACCGATATCAGTTATCCCATGTTCCAGTTGCC 19 R2       EcoRV FBP1³⁷²⁻⁶⁴⁴ FBP1- AAGCTTGCGGCCGCGATGCCACCTGGTGGACTACAG 20 F2       NotI FBP1- GGTACCGATATCAGTTATTGGCCCTGAGGTGC 18 R1       EcoRV FBP1¹⁻⁴⁴³ FBP1- AAGCTTGCGGCCGCGATGGCAGACTATTCAACA 17 F1       NotI FBP1- GGTACCGATATCAGTTATATGAGTTGCCGAGCATA 21 R3       EcoRV FBP1¹⁸⁵⁻⁶⁴⁴ FBP1- AAGCTTGCGGCCGCGATGAATGCAGTTCAAGAAATCATG 22 F3       NotI FBP1- GGTACCGATATCAGTTATTGGCCCTGAGGTGC 17 R1       EcoRV FBP1¹⁸⁵⁻⁴⁴³ FBP1- AAGCTTGCGGCCGCGATGAATGCAGTTCAAGAAATCATG 22 F3       NotI FBP1- GGTACCGATATCAGTTATATGAGTTGCCGAGCATA 21 R3       EcoRV Note: The underlined nucleotides represent the recognition site of a restriction enzyme as indicated below.

-   (c) Plasmid pBacPAK8-MTEGFP-His-FBP1 carrying, amongst others, a     His-FBP1 coding sequence encoding a His-FBP1 fusion protein (i.e., a     FBP1 protein fused with His tag at the N-terminus), was previously     constructed (Huang P. N. et al. (2011), supra). For the construction     of pBacPAK8-MTEGF-P-His-FBP1¹⁻³⁷¹, a His-FBP1¹⁻³⁷¹ coding sequence     was amplified by PCR using the following primer pair, followed by     insertion into the pBacPAK8-MTEGFP-His vector kindly provided by Dr.     Tsu-An Hsu (National Health Research Institute, MiaoLi, Taiwan) at     the XbaI and KpnI sites.     -   Forward primer (with the XbaI recognition site thereof         underlined)

(SEQ ID NO: 23) 5′-GCTCTAGAATGGCAGACTATTCAACAGTGCCT-3′

-   -   Reverse primer (with the KpnI recognition site thereof         underlined)

(SEQ ID NO: 24) 5′-CGGGGTACCTCCCATGTTCCAGTTGCCTTG-3′

-   (d) Plasmid pFLAG-CMV2-FBP1 obtained in above Item (a) was adopted     as a template for the construction of various mutant pFLAG-CMV2-FBP1     plasmids, using a site-directed mutagenesis kit (Stratagene) and the     primer pairs as shown in Table 2.

TABLE 2 Mutant FBP1 SEQ coding Primer The primer's nucleotide sequence ID sequences pair (5′ → 3′) NO. FBP1- G345-362K FBP1-MF1

25 FBP1-MR1

26 FBP1- G364-380K FBP1-MF2

27 FBP1-MR2 ATTAAATTCCTGTAGTTTTTTAGGTGGTTTCAT 28 GTTCCAGTTTTTTTGTTTTCTACCTCTTCC FBP1-G364K FBP1-MF3

29 FBP1-MR3

30 FBP1-G366K FBP1-MF4

31 FBP1-MR4

32 FBP1-G371K FBP1-MF5

33 FBP1-MR5

34 FBP1-G374K FBP1-MF6

35 FBP1-MR6

36 FBP1-G375K FBP1-MF7

37 FBP1-MR7

38 Note: Each framed region in the nucleotide sequence of an indicted primer was designed to introduce lysine residue(s) at the mutation site(s) as indicated.

-   (e) Plasmids pFLAG-FBP1-HA and pFLAG-FBP1-G371K-HA respectively     carrying, amongst others, a FBP1-HA (human influenza hemagglutinin)     coding sequence encoding an FBP1-HA fusion protein (i.e., an FBP1     protein fused with HA at the C-terminus) and a FBP1-G371K-HA coding     sequence encoding an FBP1-G371K-HA fusion protein, were generated as     follows. The FBP1-HA and FBP1-G371K-HA coding sequences were     respectively amplified from pFLAG-CMV2-FBP1 and     pFLAG-CMV2-FBP1-G371K obtained in above Items (a) and (d) using the     following primer pair. Each of the obtained coding sequences was     then in-frame inserted at the NotI and EcoRV sites of the respective     pFLAG-CMV2 vector.     -   Forward primer (with the NotI recognition site thereof         underlined)

(SEQ ID NO: 17) 5′-AAGCTTGCGGCCGCGATGGCAGACTATTCAACA-3′

-   -   Reverse primer (with the EcoRV recognition site thereof         underlined)

5′-GGTACCGATATCAGT

AG

-   -   AGCCACCTTGGCCCTGAGGTGC-3′ (SEQ ID NO: 39)     -   in which the framed region in the nucleotide sequence represents         the HA coding sequence.

-   (f) Plasmids pFLAG-CMV2-FBP1, pFLAG-CMV2-FBP1¹⁻³⁷¹ and     pFLAG-CMV2-FBP1-G371K obtained in above Items (a), (b) and (d)     respectively served as templates for the construction of recombinant     plasmids pFLAG-Hr-FBP1^(WM), pFLAG-CMV2-FBP1^(1-371-WM) and     pFLAG-Hr-FBP1-G371K^(WM), using a site-directed mutagenesis kit and     a primer pair as shown in Table 3. The obtained recombinant plasmids     pFLAG-Hr-FBP1^(WM), pFLAG-Hr-FBP1^(1-371-WM) and     pFLAG-Hr-FBP1-G371K^(WM) respectively carried a wobble mutant FBP1     (FBP1^(WM)) coding sequence, an FBP1^(1-371-WM) coding sequence (SEQ     ID NO: 44) and an FBP1-G371K^(WM) coding sequence that are resistant     to the targeting of shFBP1 and encode a respective one of FBP1^(WM),     FBP1^(1-371-WM) and FBP1-G371K^(WM).

-   (g) Plasmid pFLAG-Hr-FBP1^(WM) obtained in above Item (f) served as     a template for the construction of recombinant plasmid     pFLAG-Hr-FBP1^(1-371-WM-delNLS), which contains an     FBP1^(1-371-WM-delNLS) coding sequence (SEQ ID NO: 45) encoding     FBP1^(1-371-WM) with a deletion of nuclear localization signal (NLS)     sequence at a region spanning amino acid residues 63-78 (i.e.,     FBP1^(1-371-WM-delNLS) having an amino acid sequence of SEQ ID NO:     46). The FBP1^(1-371-WM-delNLS) coding sequence was amplified from     pFLAG-Hr-FBP1^(WM) with Nested PCR using two primer pairs     (FBP1-F1/FBP1-WMR2 and FBP1-WMF2/FBP1-R1) as shown in Table 3,     followed by in-frame insertion between the NotI and EcoRV sites of     the pFLAG-CMV2 vector.

TABLE 3 Wobble mutant SEQ FBP1 coding Primer The primer's nucleotide sequence ID sequences pair (5′→3′) NO. FBP1^(WM), FBP1-WMF1 CCATTCCTAGGTTCGCAGTCGGTATAGTTATAGGA 40 FBP1-G371K^(WM) FBP1-WMR1 TCCTATAACTATACCGACTGCGAACCTAGGAATGG 41 and FBP1^(1-371-WM) FBP1^(1-371-WM-delNLS) FBP1-F1 AAGCTTGCGGCCGCGATGGCAGACTATTCAACA 17        NotI FBP1-WMR2 AGTCATTTTGAGGAGCTCCCCCATAACCATAG 42 FBP1-WMF2 CTATGGTTATGGGGGAGCTCCTCAAAATGACT 43 FBP1-R1 GGTACCGATATCAGTTATCCCATGTTCCAGTTGCC 19        EcoRV Note: The underlined nucleotides represent the recognition site of a restriction enzyme as indicated below. 5. FBP2-Related Recombinant Plasmids

-   (a) Plasmid pFLAG-CMV2-FBP2 carrying, amongst others, an FBP2     optimized coding sequence (SEQ ID NO: 5) encoding a wild-type FBP2     protein having an amino acid sequence of SEQ ID NO: 6 (NCBI     accession no. NP_003676.2), was generated as follows. The FBP2     optimized coding sequence was optimized from the DNA of FBP2     provided by Douglas L. Black (University of California, Los Angeles)     and then amplified by PCR using a primer pair as shown in Table 4.     Nucleotides 1 to 890 of FBP2 were optimized using GeneART to     decrease GC content without changing the amino acids (Chen L. L. et     al. (2013), supra). The obtained coding sequence was subcloned     between the EcoRI and EcoRV sites of a pFLAG-CMV2 vector. -   (b) Plasmid pFLAG-CMV2-FBP2 obtained in above Item (a) was adopted     as a template for the construction of eight mutant recombinant     plasmids, including pFLAG-CMV2-FBP2-K109R, pFLAG-CMV2-FBP2-K121R,     pFLAG-CMV2-FBP2-K122R, pFLAG-CMV2-FBP2-K251R, pFLAG-CMV2-FBP2-K628R,     pFLAG-CMV2-FBP2-K646R, pFLAG-CMV2-FBP2-K654R and     pFLAG-CMV2-FBP2-K121,122R, using a site-directed mutagenesis kit and     primer pairs as shown in Table 4. Plasmid     pFLAG-CMV2-FBP2-K109,121,122R was similarly constructed except that     the template and the primer pair used were pFLAG-CMV2-FBP2-K121,122R     and primers FBP2-F109/FBP2-R109. Also, plasmid     pFLAG-CMV2-FBP2-N-ter5K5R was similarly constructed except that the     template used was pFLAG-CMV2-FBP2-K109,121,122R, and that two primer     pairs including primers FBP2-F71/FBP2-R71 and FBP2-F87/FBP2-R87 were     applied.

TABLE 4 (Wild-type or Mutant) FBP2 Primer SEQ ID coding sequence pair The primer's nucleotide sequence (5′ → 3′) NO. FBP2 FBP2-F1 ACCGAATTCGCCACCATGAGCGACTACAGCAC 47 (SEQ ID NO: 5,     EcoRI encoding an FBP2-R1 GTACCGATATCAGTTGAGCCTGCTGCTGTCCCT 48 amino acid       EcoRV sequence of SEQ ID NO: 6) FBP2-K109R (SEQ ID NO: 9, FBP2-F109

49 encoding an amino acid FBP2-R109

50 sequence of SEQ ID NO: 10) FBP2-K121R (SEQ ID NO: 11, FBP2-F121

51 encoding an amino acid FBP2-R121

52 sequence of SEQ ID NO: 12) FBP2-K122R (SEQ ID NO: 13, FBP2-F122

53 encoding an amino acid FBP2-R122

54 sequence of SEQ ID NO: 14) FBP2-K251R FBP2-F251

55 FBP2-R251

56 FBP2-K628R FBP2-F628

57 FBP2-R628

58 FBP2-K646R FBP2-F646

59 FBP2-R646

60 FBP2-K654R FBP2-F654

61 FBP2-R654

62 FBP2-K121,122R FBP2- F121/122

63 FBP2- R121/122

64 FBP2- K109,121,122R FBP2-F109

49 (SEQ ID NO: 7, encoding an FBP2-R109

50 amino acid sequence of SEQ ID NO: 8) FBP2-N-ter5K5R (K71,87,109, FBP2-F71

65 121,122R) FBP2-R71

66 FBP2-F87

67 FBP2-R87

68 Note: Each framed region in the nucleotide sequence of an indicted primer was designed to introduce arginine residue(s) at the mutation site(s) as indicated. 6. 2A Protease (2A^(pro))-Related Recombinant Plasmids

-   (a) Plasmid pGEX-6P-1-EV71-2A was constructed as follows. EV71 2A     protease (2A^(pro)) cDNA was amplified from the cDNA clone of EV71     (SRS Labs, Inc.) using the following primer pair, followed by     insertion into pGEX-6P-1 (GE Healthcare) at the EcoRI and NotI     sites.     -   Forward primer (with the EcoRI recognition site thereof         underlined)

(SEQ ID NO: 69) 5′-CCGGAATTCGGGAAATTTGGACAGCAG-3′

-   -   Reverse primer (with the NotI recognition site thereof         underlined)

(SEQ ID NO: 70) 5′-CACGATGCGGCCGCTCCTGCTCCATGGCTTC-3′

-   (b) Plasmid pGEX-6P-1-EV71-2A then served as a template for the     construction of mutant pGEX-6P-1-EV71-2A-C110S, which carries a     nucleic acid sequence encoding a mutant viral 2A^(pro) protein     having C110S mutation. Such construction was conducted using a     site-directed mutagenesis kit and the primer pair below.

Forward primer (SEQ ID NO: 71) 5′-CCAGGGGATTCCGGTGGCATT-3′ Reverse primer (SEQ ID NO: 72) 5′-AATGCCACCGGAATCCCCTGG-3′

General Experimental Procedures

Concerning the experimental methods and relevant techniques for DNA cloning as employed in this disclosure, such as DNA cleavage by restriction enzymes, polymerase chain reaction (PCR), DNA ligation with T4 DNA ligase, agarose gel electrophoresis, plasmid transformation, etc., reference may be made to the following textbook widely known in the art: Sambrook J. and Russell D. W. (2001), Molecular Cloning: a Laboratory Manual, 3rd ed., Cold Spring Harbor Laboratory Press, New York. Site-directed mutagenesis PCR was performed substantially according to the manufacturer's instructions. These techniques can be readily performed by those skilled in the art based on their professional knowledge and experience.

1. Virus Infection:

When the cultured cells reached 90% of confluence, infection by a virus, such as EV71 strain Tainan/4643/98 or CVB3 strain, at a given multiplicity of infection (m.o.i.), was performed in serum-free DMEM. The viruses were allowed to adsorb at 37° C. for 1 hour, after which the infected cells were washed with phosphate-buffered saline (PBS) and incubated at 37° C. in DMEM containing 2% FBS.

2. Expression and Purification of Recombinant Proteins:

The 2A^(pro) and 2A^(pro)-C110S proteins fused to a GST-tag as respectively produced by pGEX-6P-1-EV71-2A and pGEX-6P-1-EV71-2A-C110S in BL21 (DE3) E. coli cells (Yeastern Biotech, Taiwan) were purified using a GSTrap FF column (GE Healthcare, Waukesha, Wis.) according to the manufacturer's instructions. The GST-tag of the respective puried fusion protein was removed with PreScission Protease (GE Healthcare), so as to obtain 2A^(pro) and 2A^(pro)-C110S proteins.

Experimental procedures for the expression and purification of recombinant His-tagged FBP1 was reported previously in Huang P. N. et al. (2011), supra.

3. Coupled Transcription/Translation of [³⁵S] Methionine-Labeled FBP1:

To produce each of [³⁵S] methionine-labeled wild-type or mutant FBP1 proteins, a DNA fragment containing the T7 promoter and the designated coding sequence was amplified using PCR and a respective primer pair shown in Table 5 from the corresponding plasmid obtained in above Items (a) and (d) of the section entitled “4. FBP1-related recombinant plasmids” of the General Experimental Materials, and the designated protein was produced with the TNT-coupled reticulocyte lysate system (Promega, Madison, Wis.) according to the manufacturer's instructions.

In addition, each of [³⁵S] methionine-labeled truncated FBP1 proteins was generally produced as set forth above, except that the template to be applied was pFLAG-CMV2-FBP1 and the respective primer pair shown in Table 5 was used.

TABLE 5 FBP1 SEQ coding Primer The primer's nucleotide ID sequences pair sequence (5′ → 3′) NO. FBP1/ mutant FBP1- F4

73 FBP1 FBP1- CCAGCACCTCAGGGCCAATAAAAAAAAA 74 R4 AAAAAAAAAAAAAAAAAAAAAAA FBP1¹⁻³⁷¹ FBP1- F4

73 FBP1- TTATCCCATGTTCCAGTTGCC 75 R5 FBP1³⁷²⁻⁶⁴⁴ FBP1- F5

76 FBP1- CCAGCACCTCAGGGCCAATAAAAAAAAA 74 R4 AAAAAAAAAAAAAAAAAAAAAAA FBP1¹⁻⁴⁴³ FBP1- F4

73 FBP1- TTATATGAGTTGCCGAGCATA 77 R6 FBP1¹⁸⁵⁻⁶⁴⁴ FBP1- F6

78 FBP1- CCAGCACCTCAGGGCCAATAAAAAAAAA 74 R4 AAAAAAAAAAAAAAAAAAAAAAA FBP1¹⁸⁵⁻⁴⁴³ FBP1- F6

78 FBP1- TTATATGAGTTGCCGAGCATA 77 R6 Note: The T7 promoter sequence and the start codon are framed and boldfaced, respectively. 4. Immunoblot Analysis:

Protein samples were resolved in sodium dodecyl sulfate-polyacrylamide (SDS-PAGE) gels, and the separated proteins were subsequently transferred to polyvinylidene difluoride (PVDF) membranes (GE Healthcare). The blotted PVDF membranes thus obtained were blocked with tris-buffered saline (TBS) and 0.1% (vol/vol) TWEEN 20 (polyethylene glycol sorbitan monolaurate) containing 5% non-fat dry milk, followed by washing with TBST 3 times, each time for 15 min, and then probing with the indicated primary antibodies. For staining with secondary antibodies, the probed membranes were washed with TBST 3 times, each time for 15 min, and were then incubated with an HRP-conjugated anti-mouse antibody or an HRP-conjugated anti-rabbit antibody for 60 minutes at room temperature. HRP was detected using the Western Lightning Chemiluminescence Kit (PerkinElmer Life Sciences, Boston, Mass.).

5. Viral Plaque Assay:

6×10⁵ RD cells were seeded into each well of a 6-well plate and incubated for 24 h, followed by addition of 500 μL of virus diluent prepared in serum-free DMEM in duplicate to each well. The viruses were allowed to adsorb at 37° C. for 1 hour, after which the infected cells were washed with PBS. 2 mL of an agarose overlay medium (0.3% agarose in DMEM containing 2% FBS) was added to each well and the plates were kept at room temperature until the agarose overlay medium turned solid. The plates were moved to a 37° C. incubator for 2 to 4 days of incubation, and then the cells were fixed with a 3.6% formaldehyde solution at room temperature for 2 hours. The agarose overlay was removed, and then the fixed cells in each well were stained with 0.5% crystal violet for 2 min. After rinsing the stained cells with water, the viral plaques in each well were counted. The viral titer was determined by substituting the value of the viral plaques counted into the following formula (1): A=B/(C×0.5)  (1) wherein: A=viral titer (plaque forming units (pfu)/mL)

-   -   B=the viral plaques counted     -   C=the dilution factor of the virus

Example 1. Cleavage of FBP1 During EV71 Infection

To examine FBP1 expression and to analyze patterns of cleaved FBP1 in EV71-infected RD cell lysates, the following experiments were conducted.

Experimental Procedures

RD cells were infected with EV71 strain Tainan/4643/98 at a m.o.i. of 40 according to the procedures as described in the preceding section, entitled “1. Virus infection,” of the General Experimental Procedures. Mock-infected cells served as a control group.

At 2, 3, 4, 5, 6, 8 and 10 hours post-infection (h.p.i.), the infected cells were washed with PBS and lysed with IGEPAL CA630 (octylphenoxy poly(ethyleneoxy)ethanol) lysis buffer (150 mM NaCl, 1% IGEPAL CA630 (octylphenoxy poly(ethyleneoxy)ethanol), 50 mM Tris-base [pH 8.0]) for 30 minutes on ice. The resultant cell lysates were centrifuged at 10,000×g for 10 minutes at 4° C., and the supernatants were collected to serve as total protein samples.

Total protein samples in equal amount (determined by Bradford assay) at each designated time point of post-infection were subjected to immunoblot analysis according to the procedures as described in the preceding section, entitled “4. Immunoblot analysis,” of the General Experimental Procedures. For the purpose of comparison, the total protein sample from the mock-infected cells was subjected to the same analysis.

For analyzing patterns of cleaved FBP1 protein in EV71-infected RD cell lysates, the total protein samples obtained from the mock-infected and EV71-infected cell lysates at each designated time point of post-infection (i.e. 4, 6, 8 and 10 h.p.i.) were also subjected to immunoblot analysis using two specific FBP1 antibodies generated from the applicants' lab, i.e. Ab-N antibody and Ab-C antibody that respectively recognize the N-terminal region (amino acid positions 61-180) and C-terminal region (amino acid positions 293-644) of FBP1. In addition, viral 3D^(pol) protein was used as an indicator for virus infection, and actin was used as a loading control. The primary and secondary antibodies used for detecting the respective protein in this example are shown in Table 6 below.

TABLE 6 Proteins Primary antibody Secondary antibody FBP1 Mouse monoclonal anti- Amersham ECL Mouse FBP1 antibody (Cat. No. IgG, HRP-linked 611286, BD Biosciences, whole Ab (from Franklin Lakes, NJ) sheep) (Cat. No. NA931, GE Healthcare) N-terminal Mouse monoclonal anti- Amersham ECL Mouse region of FBP1 antibody (Cat. No. IgG, HRP-linked FBP1 611286, BD Biosciences, whole Ab (from Franklin Lakes, NJ) sheep) C-terminal Rabbit polyclonal anti- Amersham ECL Rabbit region of FBP1 antibody (Cat. No. IgG, HRP-linked FBP1 GTX115154, GeneTex, San whole Ab (from Antonio, TX) donkey) (Cat. No. NA934, GE Healthcare) 3D^(pol) Mouse anti-3D^(pol) Amersham ECL Mouse monoclonal antibody IgG, HRP-linked (generated by the whole Ab (from applicants) sheep) Actin Mouse monoclonal anti- Amersham ECL Mouse actin antibody (Cat. No. IgG, HRP-linked MAB1501 Millipore, whole Ab (from Billerica, MA) sheep) Results:

FIGS. 1 and 2 each illustrate immunoblot results showing expression of FBP1 in mock-infected cells and in EV71-infected cells at various designated time points of post-infection.

As shown in FIG. 1, decreased FBP1 expression was evident at 4 h.p.i., and the FBP1 level was significantly reduced at 8 and 10 h.p.i. In addition, a 38-kDa protein band, likely a cleavage product (Cp) of FBP1, appeared at 4 h.p.i. and reached a maximal level at 6 h.p.i., and another potential 30-kDa Cp of FBP1 appeared at 8 and 10 h.p.i., demonstrating that EV71 infection destabilizes FBP1.

As shown in FIG. 2, FBP1 protein (72 kDa) could be cleaved by 2A^(pro) into two fragments, i.e. a 38-kDa (Cp-N) FBP1 cleavage product appearing at 4 h.p.i. and a 33-kDa (Cp-C) FBP1 cleavage product appearing at 6 h.p.i., which were respectively recognized by the Ab-N and Ab-C antibodies. In addition, a caspase-induced cleavage product from Cp-N (represented by CASP-Cp-N) was observed at 8 h.p.i.

Moreover, to further determine the subcellular localization of FBP1 in EV-71-infected cells, nuclear and cytoplasmic protein fractions were isolated from the total protein sample of the infected cells, and then subjected to immunoblot analysis. The results indicated that FBP1 remained primarily in the nucleus during mock infection, but appeared in the cytoplasm with an increasing level over 2 to 6 h.p.i. In contrast, the cleavage products of FBP1 were mostly present in the cytoplasm, and their expression levels increased throughout the course of infection (data not shown).

Taken together, these results demonstrate that the FBP1 protein is likely to be subjected to proteolytic cleavage during the middle stage of EV71 infection.

Example 2. Cleavage of FBP1 by EV71 Viral Proteinase 2A In Vitro

To determine whether FBP1 cleavage was caused by viral factors, [³⁵S] methionine-labeled FBP1 generated according to the procedures as described in the section, entitled “3. Coupled transcription/translation of [³⁵S] methionine-labeled FBP1,” of the General Experimental Procedures was used in the following experiment.

Experimental Procedures

A. In Vitro Proteinase Cleavage Assay with [³⁵S] Methionine-Labeled FBP1:

5 μL of the [³⁵S] methionine-labeled FBP1 protein was incubated with various doses (0, 0.5, 1, 2, 5 and 10 μg) of EV71 viral wild-type 2A proteinase (2A^(pro)) or incubated with 10 μg of a mutant 2A proteinase (2A^(pro)-C110S) in a cleavage buffer (50 mM Tris-HCl, 50 mM NaCl, 5 mM DTT and 1 mM EDTA; pH 7.5) with a total volume of 15 μL at 37° C. for 4 hours. The reaction products were analyzed by SDS-PAGE. After the electrophoresis, gel was removed from the electrophoresis apparatus and dried by Model 583 gel dryers (Bio-Rad) for 4 hours, and then the dried gel was loaded into an X-ray cassette with an X-ray film (Amersham Hyperfilm MP) in a darkroom for overnight exposure. Following the exposure, and the X-ray film was removed from the cassette for developing.

The applicants further examined the cleavage kinetics of FBP1 by incubating 5 μg of 2A^(pro) with [³⁵S] methionine-labeled FBP1, and then assayed the cleavage result at various time points (i.e., 15 min, 30 min, 1 hour, 2 hours and 4 hours). [³⁵S] methionine-labeled FBP1 incubated with 5 μg of 2A^(C110S) and without 2A^(pro), and [³⁵S] methionine-labeled FBP1 incubated without 5 μg of 2A^(C110S) and without 2A^(pro), were used as negative control groups.

Results:

FIG. 3 shows the cleavage patterns of FBP1 by various doses of EV71 viral 2A^(pro). As shown in FIG. 3, FBP1 was cleaved by 2A^(pro) in a dose-dependent manner that resulted in two major Cps, a 38-kDa Cp designated as Cp-N, and a 33-kDa Cp designated as Cp-C.

In addition, it was found that Cp-N and Cp-C were detected after 15 minutes of incubation with 2A^(pro), and the levels of Cp-N and Cp-C increased over the 4 hours of incubation as observed (data not shown). These results provide evidence that the EV71 viral 2A^(pro) is capable of cleaving FBP1.

Example 3. Mapping of the EV71 2A^(pro) Cleavage Site in FBP1

Previous studies on picornaviruses have revealed that 2A^(pro) preferentially cuts at glycine residues (Blom N. et al. (1996), Protein Sci., 5:2203-2216; Hellen C. U. et al. (1992), J Virol., 66:3330-3338), and the result in FIG. 2 indicated that FBP1 was cleaved into two fragments, the 38 kDa N-terminal cleavage product and the 33 kDa C-terminal cleavage product. Based on this observation, the applicants predicted that 2A^(pro) likely cuts at a glycine residue located among amino acid residues 345-380 of FBP1, as shown in Table 7 below. Therefore, to pinpoint the 2A^(pro) primary cleavage site in FBP1 and to further confirm that the cleavage occurs during EV71 infection, the following experiments were conducted.

Experimental Procedures

A. In Vitro Proteinase Cleavage Assay with [³⁵S] Methionine-Labeled Wild-Type FBP1 and Mutant FBP1 Proteins:

[³⁵S] methionine-labeled wild-type FBP1 protein and seven mutant FBP1 proteins labeled with [³⁵S] methionine as shown in Table 7 were generated according to the procedures as described in the section, entitled “3. Coupled transcription/translation of [³⁵S] methionine-labeled FBP1,” of the General Experimental Procedures.

TABLE 7 Mutation sites among amino acid residues 345-380 of the respective FBP1 protein mutant FBP1 protein Wild-type SVQAGNPGGPGPGGRGRGRGQGNWNMGPPGGLQEFN FBP1 FBP1-G345- SVQA K NP KK P K P KK R K R K RGQGNWNMGPPGGLQEFN 362K FBP1-G364- SVQAGNPGGPGPGGRGRGR K Q K NWNM K PP KK LQEFN 380K FBP1-G364K SVQAGNPGGPGPGGRGRGR K QGNWNMGPPGGLQEFN FBP1-G366K SVQAGNPGGPGPGGRGRGRGQ K NWNMGPPGGLQEFN FBP1-G371K SVQAGNPGGPGPGGRGRGRGQGNWNM K PPGGLQEFN FBP1-G374K SVQAGNPGGPGPGGRGRGRGQGNWNMGPP K GLQEFN FBP1-G375K SVQAGNPGGPGPGGRGRGRGQGNWNMGPPG K LQEFN Note: The mutation sites are underlined; and the location of the predicted 2A^(pro) cleavage sites are boldfaced.

These obtained [³⁵S] methionine-labeled proteins were subjected to in vitro proteinase cleavage assay using 10 μg of 2A^(pro) at 37° C. for 4 hours according to the procedures as described in Example 2.

Results:

It can be seen from FIG. 4 that in vitro 2A^(pro) cleavage occurs on mutant FBP1-G345-362K with all the glycine residues mutated to lysine residues in the region spanning amino acid residues 345 to 362 but not on mutant FBP1-G364-380K with all the glycine residues mutated to lysine residues in the region spanning amino acid residues 364 and 380, indicating that the proteinase cleavage site is located within the region spanning amino acid residues 364 and 380. In addition, in comparison with FBP1-G364K, FBP1-G366K, FBP1-G374K and FBP1-G375K, FBP1-G371K could not be cleaved by 2A^(pro), as shown by the absence of Cp-N and Cp-C cleavage products. These results indicate that Gly-371 residue of FBP1 is likely the primary cleavage site for EV71 viral proteinase 2A.

It should be noted that regarding FBP1-G345-362K, FBP1-G364-380K, and FBP1-G371K, there are two additional non-specific cleavage products with a higher molecular weight (indicated by asterisks) as compared to Cp-N and Cp-C fragments. These additional cleavage products were unlikely to represent the cleavage products or cleavage intermediates of 2A^(pro), since they were not observed in the 2A^(pro) cleavage profile of FBP1, as shown in FIG. 3. The applicants speculated that these non-specific cleavage products might have resulted from the introduction of glycine-to-lysine mutation, which may cause: either (i) the blockage of the primary cleavage site that forces 2A^(pro) to cleave at alternative locations of FBP1, or (ii) conformational changes in FBP1 that lead to the exposure of alternative non-favored cleavage sites to 2A^(pro).

B. In Vitro Proteinase Cleavage Assay with [³⁵S] Methionine-Labeled Wild-Type FBP1 and Truncated FBP1 Proteins:

In order to confirm whether Gly-371 is the only cleavage site of 2A^(pro), [³⁵S] methionine-labeled wild-type FBP1 protein and five truncated FBP1 proteins labelled with [³⁵S] methionine (i.e., FBP1¹⁻³⁷¹, FBP1³⁷²⁻⁶⁴⁴, FBP1¹⁻⁴⁴³, FBP1¹⁸⁵⁻⁶⁴⁴ and FBP1¹⁸⁵⁻⁴⁴³) were produced according to the procedures as described in the section, entitled “3. Coupled transcription/translation of [³⁵S] methionine-labeled FBP1”, of the General Experimental Procedures, followed by conducting in vitro proteinase cleavage assay as described above in section A of this example. FIG. 5 illustrates a schematic representation of wild-type FBP1 protein and the five truncated FBP1 proteins, together with the proposed primary cleavage site at Gly-371 (indicated by an arrow) and the molecular weights of the cleavage products for wild-type FBP1 protein and each truncated FBP1 protein.

Results:

As shown in FIG. 6, both truncated FBP1¹⁻³⁷¹ and FBP1³⁷²⁻⁶⁴⁴ proteins were not cleaved by 2A^(pro), and the molecular weights thereof were respectively 38 kDa and 33 kDa, which were consistent with those of Cp-N and Cp-C. In contrast, the remaining three FBP1 truncated proteins FBP1¹⁻⁴⁴³, FBP1¹⁸⁵⁻⁶⁴⁴ and FBP1¹⁸⁵⁻⁴⁴³ containing the Gly-371 cleavage site were cleaved by 2A^(pro). In particular, a 38-kDa cleavage product was yielded from FBP1¹⁻⁴⁴³ (see lane 8), whereas both truncated FBP1¹⁸⁵⁻⁶⁴⁴ and FBP1¹⁸⁵⁻⁴⁴³ generated a similar 19-kDa cleavage product (see lanes 12 and 14), and FBP1¹⁸⁵⁻⁶⁴⁴ also yielded a 33-kDa product (see lane 12). Collectively, the cleavage patterns of wild-type and truncated FBP1 proteins indicate that Gly-371 residue of FBP1 might be the sole cleavage site of 2A^(pro).

C. The Cleavage Profiles of Wild-Type FBP1 or Mutant FBP1-G371K in EV71-Infected RD Cells:

To further confirm that the cleavage of FBP1 occurs in vivo during EV71 infection, expression of FLAG-HA dual-tagged wild-type FBP1 and mutant FBP1-G371K (resistant to 2A^(pro) cleavage) in RD cells was carried out, followed by EV71 infection.

To be specific, RD cells were transfected with pFLAG-FBP1-HA or pFLAG-FBP1-G371K-HA, and were then infected with EV71 according to the procedures as described in the preceding section, entitled “1. Virus infection,” of the General Experimental Procedures.

At 4, 6, 8 and 10 h.p.i., the cell lysates from the mock-infected and EV71-infected cells were processed according to the procedures as described in Example 1. The total protein samples thus obtained were subjected to immunoblotting analysis according to the procedures as described in the preceding section, entitled “4. Immunoblot analysis,” of the General Experimental Procedures. The primary and secondary antibodies used for detecting the respective protein in this example are shown in Table 8 below.

TABLE 8 Proteins Primary antibody Secondary antibody FLAG Mouse anti-FLAG M2 Amersham ECL Mouse monoclonal antibody IgG, HRP-linked (Cat. No. F3165, Sigma, whole Ab (from St Louis, MO) sheep) HA Mouse anti-HA monoclonal Amersham ECL Mouse antibody IgG, HRP-linked (Cat. No. H9658, Sigma, whole Ab (from St Louis, MO) sheep) EV71 3D^(pol) Mouse anti-3D^(pol) Amersham ECL Mouse monoclonal antibody IgG, HRP-linked (generated by the whole Ab (from applicants) sheep) Actin Mouse anti-actin Amersham ECL Mouse antibody (Cat. No. IgG, HRP-linked MAB1501, Millipore, whole Ab (from Billerica, MA) sheep) Results:

FIG. 7 illustrates the immunoblotting results showing the expression of FBP1 protein and its cleavage products in the EV71-infected RD cells overexpressing FLAG-HA dual-tagged wild-type FBP1 protein or mutant FBP1-G371K protein. As shown in FIG. 7, during the course of infection, FLAG-FBP1-HA was cleaved, and Cp-N and Cp-C cleavage products were respectively detected by anti-FLAG and anti-HA antibodies. On the contrary, FLAG-FBP1-G371K-HA mutant was resistant to 2A^(pro) cleavage in vivo. The results confirm that FBP1 is cleaved at Gly-371 by viral 2A^(pro) during the course of EV71 infection.

Example 4. Association of FBP1, FBP11-371 and EV71 5′UTR RNA

To address the roles of cleaved FBP1 proteins in EV71 IRES activity, the following experiments were conducted.

Experimental Materials:

Plasmids pGL3-EV71 5′UTR-FLuc and pCRII-TOPO-EV71 5′UTR were constructed according to the methods described in Huang P. N. et al. (2011), supra and Lin J. Y. et al. (2009), supra.

A DNA fragment of T7-EV71 5′UTR containing T7 promoter and the EV71 5′UTR sequence (nucleotide positions 1-745 of EV71) was excised from recombinant plasmid pCRII-TOPO-EV71 5′UTR using a EcoRI restriction enzyme. In addition, a DNA fragment of T7-EV71 5′UTR linker region containing the T7 promoter and the EV71 5′UTR linker region sequence (nucleotide positions 636-745 of EV71) was amplified from pCRII-TOPO-EV71 5′UTR using a forward primer 5′-TAATACGACTCACTATAGGGCCATCCGGTGTGCAACAGGGCAAT-3′ (SEQ ID No: 79) and a reverse primer 5′-GTTTGATTGTGTTGAGGGTCA-3′ (SEQ ID NO: 80).

Each DNA fragment was transcribed into a respective RNA transcript (i.e., a respective one of EV71 5′ UTR and EV71 5′ UTR linker region RNA probes) using a MEGAscript T7 kit (ThermoFisher Scientific, San Jose, Calif.), according to the protocol recommended by the manufacturer. In addition, biotinylated RNA transcripts, i.e. biotinylated EV71 5′UTR and EV71 5′UTR linker region RNA probes, were synthesized by adding 1.25 μL of 10 mM biotin-16-UTP (Roche, Mannheim, Germany) in the transcription reaction. These RNA transcripts were purified using an RNeasy Mini kit (Qiagen, Chatsworth, Calif.).

Experimental Procedures

A. Pull Down Assay for Biotinylated EV71 5′ UTR RNA Probe with FLAG-Fused FBP1 Protein

RD cells were transfected with a respective one of pFLAG-CMV2-FBP1, pFLAG-CMV2-FBP1¹⁻³⁷¹ and pFLAG-CMV2-FBP1³⁷²⁻⁶⁴⁴. At 48 hours post-transfection, the transfected cells were washed with PBS and lysed with a lysis buffer (containing 10 mM Tris-HCl (pH 7.4), 1 mM MgCl₂, 1 mM EGTA, 0.5% 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), 10% glycerol, 0.1 mM phenyl-methylsulfonyl fluoride (PMSF) and 5 mM β-mercaptoethanol) for 30 minutes on ice. Afterwards, the resultant cell lysate was centrifuged at 10,000×g for 10 min at 4° C., and the supernatant thus formed was collected (to serve as an RD cell extract) and stored at −80° C. for further analysis.

The obtained RD cell extract (200 μg) was mixed with 12.5 pmol of a respective one of the following: biotinylated EV71 5′UTR RNA probe, non-biotinylated EV71 5′UTR RNA probe, biotinylated EV71 5′ UTR linker region RNA probe and non-biotinylated EV71 5′ UTR linker region RNA probe, followed by adding an RNA mobility buffer (5 mM HEPES (pH 7.1), 40 mM KCl, 0.1 mM EDTA, 2 mM MgCl₂, 2 mM dithiothreitol (DTT), 1 U RNasin, and 0.25 mg/mL heparin) until a final volume of 100 μL was reached. The resultant mixture were incubated at 30° C. for 15 minutes, and then added with 400 μL of streptavidin MagneSphere Paramagnetic Particles (Promega), followed by incubation for 10 minutes at room temperature, so as to pull down the biotinylated RNA-protein complex. The obtained pulled-down complex was washed 5 times with a heparin-free RNA mobility buffer, and then 25 μL of a 2×SDS sample buffer was added to the washed complex, followed by conducting incubation at 95° C. for 10 minutes to dissociate the proteins from the complex. The eluted protein sample was subjected to immunoblotting analysis with an anti-FLAG antibody as shown in Table 8.

B. Enzymatic RNA Footprinting Assay of Wild-Type FBP1 and FBP11-371 Bound to EV71 5′ UTR Linker Region RNA

EV71 5′ UTR linker region RNA probe synthesized in this example was labeled at the 5′ end using T4 polynucleotide kinase and [γ-³²P] ATP.

The obtained ³²P-labeled EV71 5′ UTR linker region RNA probe was treated with 2 μg of wild-type FBP1 or 1.14 μg of FBP1¹⁻³⁷¹ in a binding buffer containing 1 μL of 0.02 μg/μL RNAse A (a pyrimidine nucleotide-specific endonuclease, Ambion AM2274, Thermo Fisher Scientific) or 0.02 U/mL RNAse T1 (a ribonucleotide that specifically degrades single-stranded RNA at G residues, Ambion AM2283, Thermo Fisher Scientific) at 4° C. for 10 minutes, so as to degrade the RNA sequence without the protection of a bound protein. The RNA probe without incubation of an FBP1 protein served as a control group.

Reactions were terminated with 10 μL of an inactivation buffer (Ambion, Austin, Tex.), and the cleavage products were separated in 12% acrylamide/7M urea gels, after which the gels were dried and scanned with a phosphorimager (GE Typhoon Trio Imager Scanner). RNA nucleotide positions were determined through comparison with the Decade marker (Ambion), so as to identify the sequence protected by the wild-type FBP1 and FBP1¹⁻³⁷¹.

Results:

A. Pull Down Assay

FIG. 8 illustrates immunoblot results showing association between FLAG-tagged FBP1, FBP1¹⁻³⁷¹ and FBP1³⁷²⁻⁶⁴⁴ with biotin-labeled or unlabeled EV71 5′ UTR RNA probe and EV71 5′ UTR linker region RNA probe. As shown in FIG. 8, both FLAG-FBP1 and FLAG-FBP1¹⁻³⁷¹ incubated with biotinylated EV71 5′ UTR RNA probe or EV71 5′ UTR linker region RNA probe were pulled down by the streptavidin beads, and a parallel experiment showed that these proteins were not pulled down by the beads when non-biotinylated EV71 5′UTR RNA was used. In contrast, FLAG-FBP1³⁷²⁻⁶⁴⁴ was not bound to EV71 5′UTR RNA probe or 5′UTR linker region RNA probe. These results indicate that both FLAG-FBP1 and FLAG-FBP1¹⁻³⁷¹ can bind to the EV71 5′UTR, particularly the linker region thereof.

B. Enzymatic Footprinting Assay

FIG. 9 illustrates the result of the RNA footprinting assay (using RNase T1 and RNase A) showing a mapping association of wild-type FBP1 and mutant FBP1¹⁻³⁷¹ protein with the EV71 5′ UTR linker region RNA probe at the nucleotide sequence level. The vertical lines labeling the right side of the gel indicate nucleotides that were protected from RNase T1 and RNase A digestion in the presence of wild-type FBP1 and FBP1¹⁻³⁷¹. As shown in FIG. 9, wild-type FBP1 was able to protect the EV71 5′UTR linker region RNA at nucleotide positions 686-714 (see lanes 2 and 5), while nucleotide positions 656-674 of the same linker region RNA was protected by FBP1¹⁻³¹⁷ (see lanes 3 and 6).

Taken together, these results clearly demonstrate that wild-type FBP1 and FBP1¹⁻³⁷¹ can bind to distinct sequences of the EV71 5′UTR linker region RNA.

Example 5. Evaluation for Role of FBP1¹⁻³⁷¹ as an Additive ITAF to Enhance Viral IRES Activity

To investigate whether FBP1¹⁻³⁷¹ exhibits a comparable effect on in vitro EV71 IRES activity as wild-type FBP1, shFBP1-RD stable cells with sustained knockdown of endogeneous fbp1 gene expression were prepared and used in this example.

Experimental Materials:

Short hairpin RNA (shRNA) targeting nucleotide positions 847 to 871 of human FBP1 mRNA (shFBP1, 5′-CCAAGATTTGCTGTTGGCATTGTAA-3′, SEQ ID No: 81) and the scramble control (shNC, 5′-AATTTGCGCCCGCTTACCCAGTT-3′ SEQ ID No: 82) were respectively inserted into lentivirus vectors pLKO_TRC005 (obtained from Taiwan National RNAi Core Facility, Academia Sinica) according to the instructions of the Taiwan National RNAi Core Facility, Academia Sinica, so as to construct two pLKO_TRC005-shRNA vectors, i.e. pLKO_TRC005-shFBP1 and pLKO_TRC005-shNC.

For lentivirus preparation, 293T cells were co-transfected with pLKO_TRC005-shFBP1 or pLKO_TRC005-shNC, and the helper plasmids pMD.G and pCMVΔR8.91, using X-tremeGENE transfection reagent (Roche) and then cultured at 37° C. for 36 hr. The obtained cell culture was centrifugated under 300×g. The resultant supernatant containing viral particles was harvested.

RD cells were transduced with the obtained viral particles for 24 hours, and then subjected to selection with puromycin (5 μg/mL), so as to prepare shFBP1-RD stable cells with sustained knockdown of endogeneous fbp1 gene expression and shNC-RD stable cells.

Experimental Procedures

The plasmid pGL3-EV71 5′UTR-FLuc was constructed according to the method described in Huang P. N. et al. (2011), supra and linearized using XhoI or XbaI restriction enzymes, so as to generate EV71 5′UTR-FLuc reporter RNA.

shFBP1-RD stable cells were grown in DMEM supplemented with 10% FBS, 100 units/mL of penicillin, 100 μg/mL of streptomycin and 0.25 μg/mL of amphotericin B, followed by cultivation in an incubator with culture conditions set at 37° C. and 5% CO₂. When reaching 90% of confluence, the cultured cells were washed and scraped with PBS, and then subjected to centrifugation at 300×g for 10 minutes at 4° C. After discarding the supernatant, the cell pellet thus obtained was resuspended in 1.5× pellet volume of a hypotonic lysis buffer (10 mM HEPES-KOH, pH 7.6, 10 mM KOAc, 0.5 mM Mg(OAc)₂, 2 mM DTT, and 1× protease inhibitor cocktail [Roche]), was placed on ice for 30 minutes, and was then homogenized with a 27-gauge ½-inch needle. The thus formed cell extract was centrifuged at 10,000×g for 20 minutes at 4° C., and the supernatant (serving as an shFBP1-RD extract) was recovered and used in the following in vitro IRES-driven translation assay to determine the impact of wild-type FBP1 and FBP11371 on EV71 IRES-driven translation in vitro.

To be specific, a designated concentration (i.e., 0, 25, 50, 100 or 200 nM) of recombinant wild-type FBP1 or FBP1¹⁻³⁷¹ obtained as described in Huang P. N. et al. (2011), supra was mixed with the following components to reach a final volume of 25 μL: 60% volume of the shFBP1-RD extract obtained above, 0.25 μg of EV71 5′ UTR-Fluc RNA, 10 mM creatine phosphate, 50 μg/mL creatine phosphokinase, 79 mM KOAc, 0.5 mM Mg(OAc)₂, 2 mM DTT, 0.02 mM hemin, 0.5 mM spermidine, 20 mM HEPES-KOH (pH 7.6), 20 μM amino acid mixture (Promega), 0.4 mM ATP (Promega), and an RNase inhibitor. The mixture was incubated at 30° C. for 90 minutes, and firefly luciferase activity regarding each designated concentration of wild-type FBP1 or FBP1¹⁻³⁷¹ was measured using luciferase assay system (Promega) and a luminometer (Promega). A mixture without recombinant wild-type FBP1 or FBP1¹⁻³⁷¹ added was used as a buffer control and subjected to the same analysis, and the measured firefly luciferase activity was set as 100%.

The additive effect of wild-type FBP1 and FBP1¹⁻³⁷¹ on EV71 IRES-driven translation in vitro was also generally examined using the same procedure as mentioned above, except that a designated concentration of recombinant wild-type FBP1 (50, 100 or 200 nM), FBP1¹⁻³⁷¹ (50, 100 or 200 nM) or a combination of wild-type FBP1 and FBP1¹⁻³⁷¹ (each at 25, 50 or 100 nM) was respectively used.

Results:

FIG. 10 shows the impact of various concentrations of wild-type FBP1 and FBP1¹⁻³⁷¹ on EV71 IRES-driven translation activity in vitro. As shown in FIG. 10, adding recombinant wild-type FBP1 to an shFBP1-RD cell extract increased translation from an EV71 IRES reporter in a dose-dependent manner over the buffer control. In addition, IRES activity also increased due to the addition of recombinant FBP1¹⁻³⁷¹.

FIG. 11 illustrates the additive effect of wild-type FBP1 and FBP1¹⁻³⁷¹ on EV71 IRES-driven translation activity in vitro. As shown in FIG. 11, when both wild-type FBP1 and FBP1¹⁻³⁷¹ were added to an shFBP1-RD cell extract, EV71 IRES-driven translation was activated in a dose-dependent fashion, but more importantly, IRES activity was significantly increased at a level that was substantially higher than that seen in the reaction with wild-type FBP1 alone or FBP1¹⁻³⁷¹ alone. These results demonstrate that in the presence of wild-type FBP1, FBP1¹⁻³⁷¹ can act as an additive component to enhance EV71 IRES activity.

Example 6. Evaluation for Ability of FBP1¹⁻³⁷¹ to Increase Viral Yield of Picornavirus

To address whether the truncated FBP1 assists in affecting picornavirus (such as EV71 and CVB3) yielded in infected cells, four recombinant plasmids, including pFLAG-Hr-FBP1^(WM) (expressing shRNA-resistant FLAG-FBP1^(WM)), pFLAG-Hr-FBP1-G371K^(WM) (expressing shRNA-resistant and 2A^(pro) cleavage-resistant FLAG-FBP1-G371K^(WM)), pFLAG-Hr-FBP1^(1-371-WM) (expressing shRNA-resistant FBP1^(1-371-WM)) and pFLAG-Hr-FBP1^(1-371-WM-delNLS) (expressing shRNA-resistant FBP1^(1-371-WM-delNLS) with a deletion of nuclear localization signal (NLS) sequence from FBP1^(1-371-WM)), were used in the following experiment.

Experimental Procedures

shFBP1-RD cells obtained in Example 5 were transiently transfected with pFLAG-CMV2 to serve as a vector control, or with the recombinant plasmid pFLAG-Hr-FBP1^(WM) or pFLAG-Hr-FBP1-G371K^(WM) to rescue FBP1 protein expression. At 48 hours post-transfection, the shFBP1-RD cells transfected with the respective plasmid as mentioned above and shNC-RD cells as obtained in Example 5 (as positive control) were infected with EV71 at a m.o.i. of 40 according to the procedures as described in the preceding section, entitled “1. Virus infection,” of the General Experimental Procedures. EV71 viral titers during the course of infection, i.e. at 3, 6, 9 and 12 hours, were measured by plaque assays. In comparison, shNC-RD cells were subjected to the same analysis.

On the other hand, shFBP1-RD cells were co-transfected with: (1) pFLAG-Hr-FBP1-G371K^(WM) and pFLAG-Hr-FBP1^(1-371-WM), or (2) pFLAG-Hr-FBP1-G371K^(WM) and pFLAG-Hr-FBP1^(1-371-WM-delNLS). At 48 hours post-transfection, these two types of transfected cells, alone with the previously obtained shFBP1-RD cells transfected with pFLAG-CMV2 vector or pFLAG-Hr-FBP1-G371K^(WM), were infected with EV71 or CBV3 at a m.o.i. of 40 according to the procedures as described in the preceding section, entitled “1. Virus infection,” of the General Experimental Procedures. The EV71 and CBV3 viruses thus produced were harvested at 9 hours post-infection for determination of virus titers according to the procedures as described in the preceding section, entitled “5. Viral plaque assay,” of the General Experimental Procedures. The viral titers in shFBP1-RD cells transfected with pFLAG-CMV2 vector was set as 100%.

Statistical significance of the experimental data was analyzed by performing one-way ANOVA using Prism 6 software (GraphPad Software, San Diego, Calif.), where p<0.05 was considered to be statistically significant.

Results:

FIG. 12 illustrates a line chart of the virus titers in EV71-infected shNC cells and shFBP1-RD cells expressing the FLAG-tag vector control, FBP1^(WM) or FBP1-G371K^(WM) measured as the number of plaque forming units per sample unit volume (pfu/mL), at 3, 6, 9, and 12 hours post-infection. As shown in FIG. 12, the expression of FBP1^(WM) can partially restore viral titers at 6 and 9 hours post-infection as compared with shNC cells (with FBP1 expression) and shFBP1 cells transfected with the FLAG-tag vector control (without FBP1 expression). The expression of FBP1-G371K^(WM) can also rescue viral titers at 6 and 9 hours post-infection, but to a lesser extent compared to FBP1^(WM), indicating the additive effect of FBP1¹⁻³⁷¹ in virus growth.

FIGS. 13 and 14 respectively shows the virus titers (pfu/mL) of EV71 and CVB3 in infected shFBP1-RD cells overexpressing FLAG, FBP1-G371K^(WM), FBP1-G371K^(WM) along with FBP1^(1-371-WM), and FBP1-G371K^(WM) along with FBP1^(1-371-WM-delNLS), at 6 hours post-infection. As demonstrated in FIG. 13, the expression of FBP1-G371K^(WM) significantly increased EV71 viral yield as compared to the FLAG-tagged vector control. The coexpression of FBP1-G371K^(WM) with FBP1^(1-371-WM) or FBP1^(1-371-WM-delNLS) shows even higher increment in EV71 viral yield as compared to the FLAG control or FBP1-G371K^(WM). Similarly, it is shown in FIG. 14 that, the coexpression of FBP1-G371K^(WM) with FBP1^(1-371-WM) or FBP1-G371K^(WM) with FBP1^(1-371-WM-delNLS) shows significant increase in the CVB3 viral yield in shFBP1-RD infected cells compared to the expression of FBP1-G371K^(WM) or the FLAG-tagged vector control alone. These results indicate that the truncated FBP1, such as FBP1¹⁻³⁷¹ and FBP1^(1-371-WM-delNLS), may play a key role in EV71 virus growth and yield.

Example 7. Evaluation for Roles of K109, K121, K122 as Potential Ubiquitination Sites in FBP2

The applicants found that the ubiquitination of FBP2 is promoted by the KLHL12-based CUL3 ubiquitin E3 ligase complex (data not shown). In order to identify the main ubiquitination sites on FBP2 from those predicted via proteomics analysis (see FIG. 15), eight mutant FBP2 recombinant plasmids, including pFLAG-CMV2-FBP2-K109R, pFLAG-CMV2-FBP2-K251R, pFLAG-CMV2-FBP2-K628R, pFLAG-CMV2-FBP2-K646R, pFLAG-CMV2-FBP2-K654R, pFLAG-CMV2-FLAG-FBP2-K121,122R, pFLAG-CMV2-FBP2-N-ter-5K5R, and pFLAG-CMV2-FLAG-FBP2-K109,121,122R, along with the wild-type FBP2 recombinant plasmid pFLAG-CMV2-FBP2 were subjected to the following experiments.

Experimental Procedures

A respective one of these wild-type FBP2 and mutant FBP2 recombinant plasmids was co-transfected with a plasmid pcDNA3-HA-Ub, which expressed HA tagged-ubiquitin (HA-Ub) and was provided by Dr. Rei-Lin Kuo (Chang Gung University, Taiwan) and Dr. Chen Zhao (University of Texas at Austin, USA), into 293T cells, using X-tremeGENE transfection reagent (Roche) according to the manufacturer's instructions. 293T cells transfected with pcDNA3-HA-Ub only served as a control group.

The transfected cells were treated with 20 μM MG132 (Sigma), followed by incubation for 4 hours. The cell culture was harvested and lysed with a lysis buffer containing 5 mM of N-ethylmaleimide (Sigma). The resultant cell lysate was immunoprecipitated with anti-FLAG M2 affinity gel. The immunoprecipitated product was subjected to immunoblotting according to the procedures as described in the preceding section, entitled “4. Immunoblot analysis,” of the General Experimental Procedures, so as to detect the ubiquitinated FBP2 protein. In addition, actin was used as a loading control. The primary and secondary antibodies used for detecting the respective protein in this example are shown in Table 9 below.

TABLE 9 Proteins Primary antibody Secondary antibody FLAG-tagged FBP2 Mouse anti-FLAG M2 Amersham ECL Mouse monoclonal antibody IgG, HRP-linked whole Ab (from sheep) HA-tagged protein Mouse anti-HA Amersham ECL Mouse (i.e., ubiquitinated monoclonal antibody IgG, HRP-linked FBP2 protein) whole Ab (from sheep) Actin Mouse anti-actin Amersham ECL Mouse monoclonal IgG, HRP-linked antibody whole Ab (from sheep)

In addition, the immunoblotting results (i.e. ubiquitination levels) of wild-type FBP2 and FBP2-K109,121,122R, FBP2-K121,122R and FBP2-K109R were each normalized by that of wild-type FBP2, so as to determine the fold-changes in ubiquitin modification.

Results:

As shown in the left panel of FIG. 16, the ubiquitination level of FBP2-K109R was decreased in comparison to wild-type FBP2 (see lanes 2 and 3), however, this result was not seen for FBP2-K251R, FBP2-K628R, FBP2-K646R, or FBP2-K654R (see lanes 4-7).

In addition, it can be seen from the right panel of FIG. 16, as well as FIG. 17, that the ubiquitination levels of FBP2-K121,122R (with two lysine residues at amino acid positions 121 and 122 replaced with arginine), FBP2-N-ter-5K5R (with five lysine residues at amino acid positions 71, 87, 109, 121 and 122 replaced with arginine), and FBP2-K109,121,122R (with three lysine residues at amino acid positions 109, 121 and 122 replaced with arginine) were all decreased in comparison to wild-type FBP2. Furthermore, FBP2-K121,122R had a lower ubiquitination level as compared to FBP2-K109R, but the ubiquitination level of FBP2-K109,121,122R remained lower than that of FBP2-K121,122R. This result indicates that the ubiquitination level of mutant FBP2 decreases when the number of mutation sites at amino acid positions 109, 121 and 122 of FBP2 increases.

Therefore, the applicants contemplate that Lys109, Lys121 and Lys122 are the likely sites for KLHL12-mediated ubiquitination in FBP2.

Example 8. Assessment for Importance of Ubiquitination on FBP2 Downregulation of IRES-Driven Translation

To assess the impact of ubiquitinated FBP2 on EV71 5′ UTR RNA-driven translation and to investigate how FBP2 ubiquitination affects the viral protein synthesis, shFBP2-RD stable cells with sustained knockdown of endogeneous fbp2 gene expression were prepared and used in this example.

Experimental Materials:

shFBP2-RD stable cells with sustained knockdown of endogeneous fbp2 gene expression were prepared by the procedures similar to that of shFBP1-RD stable cells as described in Example 5, except that the short hairpin RNA (shRNA) used was shFBP2 targeting nucleotide positions 813 to 837 of human FBP2 mRNA (5′-CACATTCGTATTCTGAGATCCGTCC-3′, SEQ ID No: 83). In comparision, the pLKO.1-shLacZ control plasmid TRCN0000072224 (provided by Taiwan National RNAi Core Facility, Academia Sinica) was used to prepare shLacZ-RD stable cells without knockdown of endogenous fbp2 gene expression.

Experimental Procedures

shFBP2-RD stable cells were grown in DMEM supplemented with 10% FBS, 100 units/mL of penicillin, 100 μg/mL of streptomycin and 0.25 μg/mL of amphotericin B, followed by cultivation in an incubator with culture conditions set at 37° C. and 5% CO₂. When reaching 90% of confluence, the cultured cells were washed and scraped with PBS, and were then subjected to centrifugation at 300×g for 10 minutes at 4° C. After discarding the supernatant, the cell pellet thus obtained was resuspended in 1.5× pellet volume of a hypotonic lysis buffer (10 mM HEPES-KOH, pH 7.6, 10 mM KOAc, 0.5 mM Mg(OAc)₂, 2 mM DTT, and 1× protease inhibitor cocktail [Roche]), was placed on ice for 30 minutes, and was then homogenized with a 27-gauge ½-inch needle. The thus formed cell extract was centrifuged at 10,000×g for 20 minutes at 4° C., and the resultant supernatant (serving as an shFBP1-RD cell translation extract) was recovered and used in the following in vitro IRES-driven translation assay.

293T cells were co-transfected with (1) the recombinant plasmids pFLAG-CMV2-FBP2 and pcDNA3-HA-Ub or (2) pFLAG-CMV2-FBP2-K109,121,122R and pcDNA3-HA-Ub, using the X-tremeGENE transfection reagent. At 48 hours post-transfection, the transfected cells were harvested using a lysis buffer (50 mM Tris-HCl, pH 7.4, with 150 mM NaCl, 1 mM EDTA, and 1% Triton-X-100) and placed on ice for 30 min. The resultant cell lysate was centrifuged at 12,000×g for 10 min, and the obtained supernatant was incubated with anti-FLAG M2 affinity gel (Sigma) at 4° C. for 16 hours. After washing five times with a wash buffer (50 mM Tris-HCl, pH 7.4, with 150 mM NaCl), the immunoprecipitated complex thus obtained was eluted with 3×FLAG peptides, so as to obtain an eluted product containing a FLAG-tagged ubiquitinated FBP2 protein.

For in vitro IRES-driven translation assay, the obtained eluted FLAG-tagged FBP2 protein (250 ng) was mixed with the following components to reach a final volume of 25 μL: 15 μL of the shFBP2-RD cell translation extract obtained above, 0.25 μg of EV71 5′UTR-FLuc reporter RNA as prepared in Example 5, a translation mixture (10 mM creatine phosphate, 50 μg/mL creatine phosphokinase, 79 mM KOAc, 0.5 mM Mg(OAc)₂, 2 mM DTT, 0.02 mM hemin, 0.5 mM spermidine), 20 mM HEPES-KOH (pH 7.6), 20 μM amino acid mixture (Promega), 0.4 mM ATP (Promega) and an RNase inhibitor. The mixture thus formed was incubated at 30° C. for 90 min and then measured for firefly luciferase activity using the luciferase assay system (Promega).

The aforesaid experiments were conducted in triplicate, and the results were subjected to statistical analysis using Student's two-tailed unpaired t-test using Prism 6 software (GraphPad Software, San Diego, Calif.) where p<0.05 was considered to be statistically significant.

Results:

FIG. 18 is a bar chart showing the luciferase activity of the buffer control, ubiquitinated wild-type FBP2, and FBP2-K109,121,122R with reduced ubiquitination. As shown in FIG. 18, as compared to the buffer control, the IRES-driven translation by the ubiquitinated wild-type FBP2 was significantly decreased, whereas no significant difference was observed in FBP2-K109,121,122R with reduced ubiquitination. The result indicates that the ubiquitination of FBP2 is essential for its downregulatory effect on EV71 IRES-driven translation. In other words, the mutant FBP2 proteins having at least one of the lysine residues K109, K121 and K122 that are mutated to arginine could be effective in enhancing viral IRES-driven translation activity.

Example 9. Evaluation for Influence of Ubiquitination Status on the Capability of FBP2 to Compete Against FBP1

It was noted in Example 8 that both wild-type FBP2 and the reduced ubiquitination variant (i.e. FBP2-K109,121,122R) can associate with the EV71 5′UTR RNA. To further confirm whether the competitive capability of FBP2 against other positive ITAFs (such as FBP1) is affected by ubiquitination, the following experiments were conducted.

Experimental Procedures

RD cells were transfected with the recombinant plasmid pFLAG-CMV2-FBP2 or pFLAG-CMV2-FBP2-K109,121,122R using the X-tremeGENE transfection reagent (Roche). At 48 hours post-transfection, the transfected cells were washed with PBS and lysed with a lysis buffer (containing 10 mM Tris-HCl (pH 7.4), 1 mM MgCl₂, 1 mM EGTA, 0.5% CHAPS, 10% glycerol, 0.1 mM PMSF and 5 mM β-mercaptoethanol) for 30 minutes on ice. Afterwards, the resultant cell lysate was centrifuged at 10,000×g for 10 min at 4° C., and the supernatant thus formed was collected (serving as an RD cell extract) and stored at −80° C. for further analysis.

The obtained RD cell extract (200 μg) was mixed with the following components to reach a final volume of 100 μL: 12.5 pM of the biotinylated EV71 5′UTR RNA probe as obtained in Example 4, different concentrations (0, 0.5, 1 and 2 μM) of recombinant His-tagged wild-type FBP1, and an RNA mobility shift buffer (5 mM HEPES (pH 7.1), 40 mM KCl, 0.1 mM EDTA, 2 mM MgCl₂, 2 mM DTT and 0.25 mg/mL heparin). After incubation at 30° C. for 15 min, the thus formed mixture containing protein-biotinylated RNA complexes was added to 400 μL of Streptavidin MagneSphere Paramagnetic Particles (Promega) for capturing at room temperature for 10 min. The captured product thus obtained was washed five times with a heparin-free RNA mobility shift buffer, after which 30 μL of a 2× sample buffer was added, followed by incubation for 10 min at room temperature to dissociate the protein-biotinylated RNA complexes. The thus formed sample containing the eluted protein was incubated at 95° C. for 5 min and then subjected to immunoblotting according to the procedures as described in the preceding section, entitled “4. Immunoblot analysis,” of the General Experimental Procedures. The primary and secondary antibodies used for detecting the respective protein in this example are shown in Table 10 below.

TABLE 10 Proteins Primary antibody Secondary antibody FLAG-tagged Mouse anti-FLAG M2 Amersham ECL Mouse FBP2 monoclonal antibody IgG, HRP-linked (Cat. No. F3165, Sigma- whole Ab (from Aldrich, St Louis, MO) sheep) His-tagged Mouse anti-His Amersham ECL Mouse FBP1 monoclonal antibody IgG, HRP-linked (Cat. No. OB05, whole Ab (from CalBioChem, LaJolla, sheep) CA) Results:

FIG. 19 shows the result of the immunoblot assay evaluating the EV71 5′ UTR RNA competition dynamics for wild-type FBP2, FBP2-K109,121,122R and the positive ITAF FBP1. As shown in FIG. 19, increasing amounts of FBP1 competed against both wild-type FBP2 and FBP2-K109,121,122R for binding to EV71 5′ UTR RNA. In addition, under the same amount of FBP1, FBP2-K109,121,122R was more significantly outcompeted for binding to the EV71 5′ UTR as compared to wild-type FBP2. On the other hand, a reverse competition assay using wild-type FBP2 or FBP2-K109,121,122R against FBP1 was also conducted, and the results suggest that increasing levels of wild-type FBP2 and FBP2-K109,121,122R outcompeted FBP1 for binding to EV71 5′UTR RNA (data not shown). Taken together, these results demonstrate that reduction of FBP2 ubiquitination may diminish the competitive capability of FBP2 against FBP1 for binding to EV71 5′UTR RNA, thereby suppressing the inhibitory effect of FBP2 on EV71 IRES-driven translation.

Example 10. Evaluation for Ability of Mutant FBP2 with Reduced Ubiquitination to Increase EV71 Viral Yield

In order to investigate how the ubiquitination of FBP2 affects EV71 and CVB3 virus yield in infected RD cells, the following experiments were conducted.

Experimental Procedures

A. Determination of Viral Protein Synthesis

shFBP2-RD cells obtained in Example 8 were transfected with a respective one of pFLAG-CMV2 vector as a vector control, pFLAG-CMV2-FBP2 and pFLAG-CMV2-FBP2-K109,121,122R.

At 24 hours post-transfection, 2.5×10⁵ transfected cells were seeded into each well of a 12-well plate and incubated at 37° C. for additional 24 hours. The obtained cell culture was infected with EV71 at a m.o.i. of 10 according to the procedures as described in the preceding section, entitled “1. Virus infection,” of the General Experimental Procedures. The medium was replaced with methionine-free DMEM, and incubation was continued at 37° C. for 1 hour. Afterwards, the medium was replaced with DMEM containing ^([35S])-methionine (50 μCi/mL) to label the newly synthesized viral proteins. After 1 hour of labeling, the cells were washed with PBS and lysed with a lysis buffer (150 mM NaCl, 50 mM Tris-Base, 1% IGEPAL CA630 (octylphenoxy poly(ethyleneoxy)ethanol), pH 8.0). The cell lysate was centrifuged at 10000×g for 10 min at 4° C. The resultant supernatant was subjected to SDS-PAGE, followed by transfer to a PVDF membrane and detection by autoradiography and immunoblotting.

Viral proteins seen from autoradiography were identified according to the protein size. The levels of [³⁵S]-methionine-labeled VP1 and 3C^(pro) proteins were quantified and normalized against the actin level, based on two repeated experiments.

B. Plaque Assay

shFBP2-RD cells as obtained in Example 8 were transfected with a respective one of the following plasmids: pFLAG-CMV2 as a vector control, pFLAG-CMV2-FBP2, pFLAG-CMV2-FBP2-K109,121,122R, pFLAG-CMV2-FBP2-K109R, pFLAG-CMV2-FBP2-K121R and pFLAG-CMV2-FBP2-K122R. After incubation at 37° C. for 2 days, the transfected cells were subjected to virus infection with EV71 or CVB3 virus at a m.o.i. of 40 according to the procedures as described in the preceding section, entitled “1. Virus infection,” of the General Experimental Procedures. The EV71 and CVB3 viruses thus produced were harvested at 9 hours post-infection, and the virus titers were titrated according to the procedures as described in the preceding section, entitled “5. Viral plaque assay,” of the General Experimental Procedures. The virus titer in the FLAG vector control was set as 100%.

Statistical significance of the experimental data was analyzed by performing one-way ANOVA using Prism 6 software (GraphPad Software, San Diego, Calif.), where p<0.05 was considered to be statistically significant.

Results:

FIG. 20 illustrates the relative expression levels of newly synthesized viral proteins VP1 and 3C^(pro) against the actin level in the shFBP2-RD cells overexpressing the FLAG vector control, wild-type FBP2 and mutant FBP2-K109,121,122R with reduced ubiquitination. As shown in FIG. 20, the relative expression levels of VP1 and 3C^(pro) were significantly lower in the wild-type FBP2-expressing cells as compared to those in the cells expressing the FLAG vector control and FBP2-K109,121,122R.

FIGS. 21 and 22 respectively illustrate the virus titers of EV71 and CBV3 in the shFBP2-RD cells overexpressing the respective one of the FLAG vector control, wild-type FBP2, and mutant FBP2 at 9 hours post-infection. As shown in FIG. 21, the FBP2 mutants with reduced ubiquitination, including FBP2-K109,121,122R, FBP2-K109R, FBP2-K121R and FBP2-K122R, increased the viral yield as compared to wild-type FBP2, with FBP2-K109,121,122R being the most potent in increasing EV71 viral yield.

Similarly, as revealed in FIG. 22, FBP2-K109,121,122R showed a significant increase in the CVB3 viral yield as compared to wild-type FBP2, however, no significant difference in the virus titer was observed in FBP2-K109, 121, 122R as compared to the FLAG vector control. These results thus infer that FBP2 mutants with at least one of the lysine residues K109, K121 and K122 that are mutated to arginine could be effective in enhancing viral IRES-driven translation activity and thereby increasing viral yield.

All patents and references cited in this specification are incorporated herein in their entirety as reference. Where there is conflict, the descriptions in this case, including the definitions, shall prevail.

While the disclosure has been described in connection with what are considered the exemplary embodiments, it is understood that this disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements. 

What is claimed is:
 1. A recombinant nucleic acid encoding a recombinant polypeptide having an amino acid sequence corresponding to that of a truncated mutant product of a wild-type FBP1 protein that comprises the amino acid sequence of SEQ ID NO: 2, the truncated mutant product lacking a C-terminal domain of the wild-type FBP1 protein, the recombinant nucleic acid having a nucleotide sequence selected from SEQ ID NO: 3, SEQ ID NO: 44 and SEQ ID NO:45.
 2. A recombinant expression vector comprising a recombinant nucleic acid of claim
 1. 3. A recombinant cell comprising a recombinant expression vector of claim
 2. 4. The recombinant cell of claim 3, wherein the recombinant nucleic acid of the recombinant expression vector is introduced into a genome of the recombinant cell.
 5. A recombinant nucleic acid encoding a recombinant polypeptide having an amino acid sequence corresponding to that of a mutant product of a wild-type FBP2 protein that comprises the amino acid sequence of SEQ ID NO: 6, each of amino acid residues at positions 109, 121 and 122 of the wild-type FBP2 protein being lysine, at least one of amino acid residues at positions 109, 121 and 122 of the mutant product being arginine instead of lysine.
 6. The recombinant nucleic acid of claim 5, which has a nucleotide sequence selected from the group consisting of SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11, SEQ ID NO: 13 and SEQ ID NO:
 15. 7. A recombinant expression vector comprising a recombinant nucleic acid of claim
 5. 8. A recombinant cell comprising a recombinant expression vector of claim
 7. 9. The recombinant cell of claim 8, wherein the recombinant nucleic acid of the recombinant expression vector is introduced into a genome of the recombinant cell.
 10. The recombinant cell of claim 8, wherein an endogenous FBP2 gene in the genome of the recombinant cell is deleted, disrupted, or disabled.
 11. The recombinant cell of claim 8, which further comprises a recombinant expression vector including a recombinant nucleic acid encoding a recombinant polypeptide having an amino acid sequence corresponding to that of a truncated mutant product of a wild-type FBP1 protein having 644 amino acids in length, wherein the truncated mutant product lacks a C-terminal domain of the wild-type FBP1 protein. 